WASHINGTON  Environmental groups and a dozen states will argue the U.S. government should regulate emissions of greenhouse gases that spur global warming in a pivotal case before the Supreme Court Wednesday.

Plaintiffs in the case, which is known as Massachusetts v. EPA, contend the U.S. Environmental Protection Agency's mandate to shield Americans from harmful pollution includes putting limits on car, truck and power plant emissions that have been shown to hasten climate change.

But the EPA, along with 10 states, four motor vehicle trade associations and two coalitions of utility companies and other industries maintain the agency lacks the authority to limit emissions of greenhouse gases such as carbon dioxide.

At the heart of the case is a dispute over whether greenhouse gases fit within the federal Clean Air Act's definition of a pollutant. If they do, the plaintiffs argue the EPA then has the authority to regulate them.

Greenhouse gases occur naturally and are also emitted by cars, trucks and factories into the atmosphere. They can trap heat close to Earth's surface like the glass walls of a greenhouse.

Emissions have risen steeply over the past century and many scientists see a connection between this rise and an increase in global average temperatures and a related increase in extreme weather, wildfires, melting glaciers and other damage to the environment.

This case is not a debate about whether these emissions are linked to global warming. The Bush administration, and in fact President Bush himself, have acknowledged this link, and Bush told a summit of industrialized nations this year that human activities play a role in world climate change.

ARE GREENHOUSE GASES POLLUTANTS?

At issue is whether the U.S. government has the power to cap these emissions. Industry groups argue that it doesn't, and that carbon dioxide is a naturally occurring gas that does not fit the U.S. Clean Air Act's definition of a pollutant.

The Bush administration has consistently rejected capping greenhouse gas emissions as bad for business and U.S. workers.

In his first year as president in 2001, Bush withdrew from the Kyoto Protocol, an international agreement aimed at cutting greenhouse gases by setting limits on emissions from industrialized nations.

Bush called this agreement "unrealistic" and offered an alternative plan offering incentives for voluntary emissions cuts.

This is at odds with contentions by the plaintiffs in the case: Massachusetts, California, Connecticut, Illinois, Maine, New Jersey, New Mexico, New York, Oregon, Rhode Island, Vermont and Washington state; the cities of Washington D.C., New York and Baltimore; the government of American Samoa, and 13 environmental groups.

Those arguing against putting limits on greenhouse gas emissions are: the EPA, 10 states -- Michigan, Texas, Idaho, North Dakota, Utah, South Dakota, Alaska, Kansas, Nebraska and Ohio -- and numerous industry groups, including the Alliance of Automobile Manufacturers, which represents nine car makers including the so-called "Big Three".

Both sides are to make oral arguments Wednesday before the Supreme Court. A ruling is expected by the middle of next year.